1. Field of the Invention
This invention relates to the field of electronic article surveillance systems, and in particular, to optimizing transmitter to antenna coupling for interlaced transmitter phases.
2. Description of Related Art
Electronic article surveillance (EAS) systems employ magnetic markers, also referred to as tags, which are placed on articles or products which are monitored to prevent unauthorized removal from a restricted space, for example a retail store or a library. Egress from the space is restricted to a lane or path into which a radio frequency interrogating signal is transmitted. This area is referred to as the interrogation zone. If the marker or tag is present in or on the article, and the marker or tag has not been deactivated, the marker or tag acts as a transponder and generates a return signal which can be identified by a receiver. The receiver can initiate an audible alarm, for example, or trigger other protective measures.
The transmitting and receiving antennas, often referred to as the transmitter/receiver pair, are mounted in floors, walls, ceilings or free standing pylons. These are necessarily fixed mounting positions. The articles, on the other hand, may be carried through the field of the interrogating signal in any orientation, and accordingly, so may the tags or markers.
The two most common antenna configurations are a rectangular loop and a "figure-8". These are implemented by using two adjacent rectangular loops, as shown in FIGS. 5(a) and 5(b). In FIG. 5(a) a pylon structure P has an upstanding portion on which two rectangular transmitting loops A and B are mounted with adjacent legs at height h above the floor. When the loops are driven by current flowing in the same direction, for example clockwise as indicated by arrows I.sub.A and I.sub.B in FIG. 5(a), the current D in the bottom leg of loop A and the current E in the top leg of loop B flow in opposite directions. Accordingly, the respective fields generated by currents D and E mostly cancel out one another. The overall effect is that of a single, large rectangular loop. This is referred to as an in-phase mode of operation. When the loops are driven by current flowing in opposite directions, as indicated by arrows I.sub.A and I.sub.B in FIG. 5(b), the current D in the bottom leg of loop A and the current E in the top leg of loop B flow in the same direction. Accordingly, the respective fields generated by currents D and E reinforce one another. The overall effect is that of a single, large "figure-8" loop. This is referred to as a "figure-8" or out-of-phase mode of operation. It will be appreciated that the two loop configurations can have shapes other than strictly rectangular, for example oval.
A single rectangular loop transmitter, the in-phase configuration, will provide substantial horizontal magnetic field, but a significantly lower or even zero valued vertical component, especially at the central height h of the interrogation zone. On the other hand, if a "figure 8" transmitter configuration is used, the vertical magnetic field becomes stronger but the horizontal component becomes weaker or even zero valued. Therefore it is desirable to interlace the transmitter phases, that is, alternate transmissions from the two antenna configurations, to maximize the system performance for all orientations of markers in the interrogation zone.
However, driving two transmitter loops in both the in-phase and figure-8 configurations requires different resonant capacitors to achieve the proper resonant conditions for each of the two modes. There is a significant difference in the resonant frequency, normally about 3 kHz, between the two antenna phases. When the transmitter is off-resonant, not enough current can be injected into the transmitter as is required for proper system detection.
An ULTRA MAX.RTM. marker or tag is the kind of tag having two components. One component is an amorphous material which responds to an interrogating signal at a resonant frequency, for example 58 KHz, in the presence of a magnetic bias. The other component is a magnetic material which provides the magnetic bias making possible the resonant response of the amorphous material. As may be expected, there is a distribution of manufactured marker frequencies due to process and material fluctuation. The marker frequency also varies with magnetic field. The resonant frequency of a linear ULTRA MAX.RTM. marker can shift up or down by about three to four hundred Hz in the vertical orientation due to the earth's magnetic field. The term ULTRA MAX.RTM. is a registered trademark of Sensormatic Electronics Corporation. Therefore, it is also desirable to transmit two frequencies, instead of one frequency, to increase the effective peak performance of the marker. The additional frequencies chosen are typically about two to three hundred Hz from the center operating frequency. Consequently, the transmitter of such a dual frequency system can not be optimized.
Accordingly, there has been a long felt need to provide an interlaced, dual frequency EAS system which can be optimized for peak performance and reliability.